1. Field of the Invention
The present invention relates generally to reconstitutable fuel assemblies for nuclear reactors and, more particularly, is concerned with a top nozzle incorporating an alignment sleeve capture arrangement which eliminates all loose components during top nozzle removal and replacement in carrying out reconstitution of the fuel assembly.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle.
Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of coil springs surrounding the guide thimbles, such as seen in U.S. Pat. Nos. 3,770,583 and 3,814,667 to Klumb et al and 4,269,661 to Kmonk et al, and in the first three patent applications cross-referenced above. Also, as exemplified by the fourth patent application cross-referenced above, recently it has been proposed that leaf springs be used in place of the coil springs.
Due to occasional failure of some fuel rods during normal reactor operation and in view of the high costs associated with replacing fuel assemblies containing failed fuel rods, the trend is currently toward making fuel assemblies reconstitutable in order to minimize operating and maintenance expenses. Conventional reconstitutable fuel assemblies incorporate design features arranged to permit the removal and replacement of individual failed fuel rods. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened in some instances, such as illustrated in the above cross-referenced applications, by a threaded arrangement to the upper end of each control rod guide thimble, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in a neutron-absorbing liquid. Once removal and replacement of the failed fuel rods have been carried out on the irradiated fuel assembly submerged at a work station and after the top nozzle has been remounted on the guide thimbles of the fuel assembly, the reconstituted assembly can then be reinserted into the reactor core and used until the end of its usefuel life.
While the proposed removable top nozzle designs of the above cross-referenced patent applications have proved to be highly satisfactory ways to facilitate reconstitution of fuel assemblies, the top nozzle arrangement of the fourth cross-referenced application does require handling of loose parts for disassembly and replacement of the top nozzle, namely, the enlarged nuts threaded on the upper end portions of the guide thimbles which upper end portions can take the form of alignment sleeves threaded on the guide thimbles. Consequently, a need exists for a different approach to top nozzle attachment, one with the objective of removing and reattaching the top nozzle without creating any loose parts and yet maintains all of the desirable features of both the leaf spring and coil spring hold-down device designs of the above cross-referenced patent applications.